In the image forming apparatus of the electrophotographic type, or the like, the toner image is fixed on the recording material by heating and pressing the toner image formed on the recording material by a fixing device (image heating apparatus). In the fixing device, a pair of rotatable members is provided, and, at a nip therebetween, fixing of the toner image is carried out.
In a case when the toner image is fixed on the recording material, heat is transferred from the rotatable members to the recording material, so that a surface temperature of the rotatable members lowers.
Therefore, a type in which the rotatable member is externally heated in addition to interval heating of the rotatable members has been proposed (Japanese Laid-Open Patent Application 2012-2926). Specifically, the rotatable member is heated using an external heating belt (endless belt) stretched by two rollers. In the two rollers, heaters are incorporated, respectively, and two temperature sensors are provided opposed to the respective rollers via the external heating belt.
In such a fixing device, in a heating region where the external heating belt and the rotatable member contact each other, heat supply from the external heating belt to the rotatable member is carried out. For that reason, there is a tendency that, as compared with a temperature of the external heating belt in an upstream side of the heating region, the temperature of the external heating belt in a downstream side of the heating region lowers.
In a constitution exhibiting such a tendency, it is required that the external heating belt is controlled to a proper temperature.